r-process

The rapid neutron capture process or r-process is a set of reactions in nuclear astrophysics that are responsible for the creation (nucleosynthesis) of approximately half the atomic nuclei heavier than iron.

This process entails a succession of rapid neutron captures (hence the name) by heavy seed nuclei, typically beginning with ⁵⁶Fe. The captures must be rapid in the sense that the nucleus does not have time to undergo radioactive decay before another neutron arrives to be captured. The r-process therefore occurs in locations where there is a high density of free neutrons. These include the material ejected from a core-collapse supernova (as part of supernova nucleosynthesis) and decompression of neutron-rich matter thrown off from a neutron star merger. The relative contributions of these, and other, sources to the astrophysical abundance of r-process elements is a matter of ongoing research.

The r-process also occurs to a minor extent in thermonuclear weapon explosions. This led to the discovery of the elements einsteinium (element 99) and fermium (element 100) in nuclear weapon fallout.

The r-process is distinguished from the s-process, the other predominant mechanism for the production of heavy elements, which is nucleosynthesis by means of slow captures of neutrons. The s-process primarily occurs within stars, particularly AGB stars, where the neutron flux is sufficient to cause reactions but too low for the r-process. The s-process is secondary, meaning that it requires pre-existing heavy isotopes as seed nuclei to be converted into other heavy nuclei. Taken together, the r- and s-processes account for the majority of abundance evolution of elements heavier than iron.

History

The need for some kind of rapid capture of neutrons was seen from the relative abundances of isotopes of heavy elements given in a newly published table of abundances by Hans Suess and Harold Urey in 1956. Radioactive isotopes must capture another neutron faster than they can undergo beta decay in order to create abundance peaks at germanium, xenon, and platinum. According to the nuclear shell model, radioactive nuclei that would decay into isotopes of these elements have closed neutron shells near the neutron drip line, where more neutrons cannot be added. Those abundance peaks created by rapid neutron capture implied that other nuclei could be accounted for by such a process. That process of rapid neutron capture in neutron-rich isotopes is called the r-process. A table apportioning the heavy isotopes phenomenologically between s-process and r-process was published in the famous B²FH review paper in 1957, which named that process and outlined the physics that guide it. B²FH also elaborated the theory of stellar nucleosynthesis and set substantial frame-work for contemporary nuclear astrophysics.

The r-process described by the B²FH paper was first computed time-dependently at Caltech by Phillip Seeger, William A. Fowler and Donald D. Clayton, who achieved the first successful characterization of the r-process abundances and showed its evolution in time. They were also able using theoretical production calculations to construct more quantitative apportionment between s-process and r-process of the abundance table of heavy isotopes, thereby establishing a more reliable abundance curve for the r-process isotopes than B²FH had been able to define. Today, the r-process abundances are determined using their technique of subtracting the more reliable s-process isotopic abundances from the total isotopic abundances and attributing the remainder to the r-process nucleosynthesis. That r-process abundance curve (vs. atomic weight) gratifyingly resembles computations of abundances synthesized by the physical process.

Most neutron-rich isotopes of elements heavier than nickel are produced, either exclusively or in part, by the beta decay of very radioactive matter synthesized during the r-process by rapid absorption, one after another, of free neutrons created during the explosions. The creation of free neutrons by electron capture during the rapid collapse to high density of the supernova core along with assembly of some neutron-rich seed nuclei makes the r-process a primary process; namely, one that can occur even in a star of pure H and He, in contrast to the B²FH designation as a secondary process building on preexisting iron.

Observational evidence of the r-process enrichment of stars, as applied to the abundance evolution of the galaxy of stars, was laid out by Truran in 1981. He and many subsequent astronomers showed that the pattern of heavy-element abundances in the earliest metal-poor stars matched that of the shape of the solar r-process curve, as if the s-process component were missing. This was consistent with the hypothesis that the s-process had not yet begun in these young stars, for it requires about 100 million years of galactic history to get started. These stars were born earlier than that, showing that the r-process emerges immediately from quickly-evolving massive stars that become supernovae. The primary nature of the r-process from observed abundance spectra in old stars born when the galactic metallicity was still small but that nonetheless contain their complement of r-process nuclei.

Periodic table showing the cosmogenic origin of each element. The elements heavier than iron with origins in supernovae are typically those produced by the r-process, which is powered by supernovae neutron bursts

This scenario, though generally supported by supernova experts, has yet to achieve a totally satisfactory calculation of r-process abundances because the overall problem is numerically formidable; but existing results are very supportive.

The r-process is responsible for our natural cohort of radioactive elements, such as uranium and thorium, as well as the most neutron-rich isotopes of each heavy element.

Nuclear physics

Immediately after the severe compression of electrons in a core-collapse supernova, beta-minus decay is blocked. This is because the high electron density fills all available free electron states up to a Fermi energy which is greater than the energy of nuclear beta decay. But nuclear capture of those free electrons still occurs, and causes increasing neutronization of matter. There results an extremely high density of free neutrons which cannot decay, and as a result a large neutron flux (on the order of 10²² neutrons per cm² per second) and high temperatures. As this re-expands and cools, neutron capture by still-existing heavy nuclei occurs much faster than beta-minus decay. As a consequence, the r-process runs up along the neutron drip line and highly-unstable neutron-rich nuclei are created.

Three processes which affect the process of climbing the neutron drip line are; a notable decrease in the neutron-capture cross section at nuclei with closed neutron shells, the inhibiting process of photodisintegration, and the degree of nuclear stability in the heavy-isotope region. This last phenomenon terminates the r-process when its heaviest nuclei become unstable to spontaneous fission, which is currently believed to be in the neutron-rich region of the table of nuclides when the total number of nucleons approaches 270; even before this, the fission barrier may be low enough that neutron capture might induce fission instead of continuing up the neutron drip line. After the neutron flux decreases, these highly unstable radioactive nuclei undergo a rapid succession of beta decays until they reach more stable, neutron-rich nuclei. So, while the s-process creates an abundance of stable nuclei having closed neutron shells, the r-process creates an abundance of nuclei about 10 amu below the s-process peaks as a result of the decay back towards stability.

The r-process also occurs in thermonuclear weapons, and was responsible for the initial discovery of neutron-rich almost stable isotopes of actinides like plutonium-244 and the new elements einsteinium and fermium (atomic numbers 99 and 100) in the 1950s. It has been suggested that multiple nuclear explosions would make it possible to reach the island of stability, as the affected nuclides (starting with uranium-238 as seed nuclei) would not have time to beta decay all the way to the quickly spontaneously fissioning nuclides at the line of beta stability before the absorbed more neutrons in the next explosion, thus providing a chance to reach isotopes like copernicium-291 and -293 which should have half-lives of centuries or millennia.

Astrophysical sites

The most probable candidate sites for the r-process has long been suggested to be core-collapse supernovae (spectral types Ib, Ic and II), which may provide the necessary physical conditions for the r-process. However, the scarcity of r-process nuclei requires that either only a small fraction of supernovae eject r-process nuclei to the interstellar medium, or that each supernova ejects only a very small amount of r-process material. In addition, the ejected material must be relatively neutron-rich, a condition which has been difficult to achieve in models.

An alternative site proposed in 1974 was decompressing neutron star matter. It was proposed such matter is ejected from neutron stars merging with black holes in compact binaries. In 1989 (and 1999) this scenario was extended to binary neutron star mergers (a binary star system of two neutron stars that collide). After preliminary identification of these sites, the scenario was confirmed in GW170817.